The present invention relates to a compressed gas storage tank utilizing a rock-bed cavity, in which a bentonite slurry (drilling mud or clay base mud) is fed into an underground cavity formed in a rock-bed, and compressed gasses such as air or natural gas, which have been forcibly fed into the underground cavity through a compressed gas pipe, is stored in the rock-bed cavity in a state in which the compressed gas is loaded with a pressure load of the bentonite slurry from the underside of the compressed gas.
For example, a compressed air storage tank (CAES) used for a compressed air storage/gas turbine power generation is constituted such that a compressed air is stored in a closed space formed in the underground.
It is, in general, difficult to construct or to install a tank for storing a compressed gas such as a compressed air or natural gas considerably deep in the underground. In view of the foregoing, compressed gas storing tanks are proposed, for example, by Japanese Patent Application Laid-Open Nos. 09-154244 and 11-153082, in which an underground cavity or a abandoned mine formed in a rock-bed, such as a self-sustaining hard rock, by means of excavation operation is utilized as it is.
On the other hand, in a tank utilizing such a rock-bed cavity, since a compressed gas tends to leak through a crack or the like formed in a rock-bed at the time of storing the compressed gas, its air-tightness needs to be improved. As a method for increasing the air-tightness of the rock-bed cavity, there can be contemplated a method for forming a concrete layer over an inner wall surface of a rock-bed cavity, a method for adhering a rubber sheet, a method for employing a water sealing tunnel, or a method in which a bentonite slurry is fed into a rock-bed cavity and a mud cake is formed on an inner wall surface by muddy component such as bentonite contained in the bentonite slurry so that a void and a crack formed in the rock-bed is blocked with this mud cake.
According to this compressed air storage tank mentioned above, a working fluid composed of a bentonite slurry and the like is supplied into an air storage chamber from the underside thereof, which chamber is a closed space for storing a compressed air, for example through a connected passageway between the air storage chamber and a storage reservoir provided at an upper location of the air storage chamber, and the working fluid is flowed between the air storage chamber and the storage reservoir in accordance with a storing quantity of the compressed air, so that the compressed air is stored in a state in which the compressed air is loaded with a hydrostatic pressure of the working fluid from the underside thereof. During night time when a compressed air is fed and stored utilizing surplus electric power, the working fluid is moved to the storage reservoir as the storing quantity of the compressed air is increased. During day time when the compressed air is used for power generation, the working fluid is moved to the air storage chamber as the storing quantity of the compressed air is reduced.
However, the conventional method for increasing the air-tightness of the rock-bed cavity utilizing a bentonite slurry has shortcomings in that air and gas tend to remain particularly at the top end part of the rock-bed cavity at the time of filling a bentonite slurry and a stable mud cake is difficult to make, thus making it unable to fully block the void and crack in some cases. Moreover, self-maintenance of air-tightness, which is performed, for example, during operation of the compressed gas storage tank, by repeatedly creating a fluid tight state utilizing a bentonite slurry and an air-tight state utilizing a compressed gas is not sufficiently performed in some cases, particularly at the top end part of the rock-bed cavity where the compressed gas tends to remain.
The present invention has been made by paying attention to such conventional problems as just mentioned above. It is, therefore, an object of the present invention to provide a compressed gas storage tank utilizing a rock-bed cavity in which a top end part of a rock-bed cavity is surely blocked to realize a sufficient air-tightness, and in which self-maintenance of air-tightness, which is performed by repeatedly creating a fluid tight state and an air-tight state, can surely be performed at the top end part of the rock-bed cavity, and therefore a compressed gas can be stored in a stable condition without allowing escape of the compressed gas.
The present invention has achieved the above object by providing a compressed gas storage tank utilizing a rock-bed cavity in which a bentonite slurry is fed into an underground cavity formed in a rock-bed, and a compressed gas forcibly fed to the underground cavity through a compressed gas pipe is stored in the rock-bed cavity in a state in which the compressed gas is loaded with a pressure load of the bentonite slurry from the underside of the compressed gas, wherein the compressed gas pipe is open downward from a top portion of the rock-bed cavity, and the bentonite slurry in the rock-bed cavity is of a dual layer structure consisting of an upper layer composed of a light bentonite slurry mixed with a filling-up material (for example, a lost circulation inhibitor) invading into and filling up a void and a crack formed in an inner wall surface of the rock-bed cavity and having a specific gravity of 1.05 to 1.20, and a lower layer composed of a heavy bentonite slurry mixed with a high specific gravity fine powder as a load control material and having a specific gravity of 1.20 to 2.0 (the invention as defined in claim 1).
Moreover, according to the compressed gas storage tank of the present invention, it is preferred that the light bentonite slurry is forcibly fed through the compressed gas pipe into the rock-bed cavity in which the bentonite slurry is filled (the invention as defined in claim 2).
Furthermore, according to the compressed gas storage tank of the present invention, it is also preferred that the rock-bed cavity is an underground cavity extending laterally with its ceiling part exhibiting a rising gradient toward the top part which is a connecting portion with the compressed gas pipe (the invention as defined in claim 3).
Moreover, according to the compressed gas storage tank of the present invention, it is also preferred that the heavy bentonite slurry is fed into the rock-bed cavity through a vertical shaft formed in the underground and the vertical shaft has associated facilities such as a reverse osmosis membrane water-generating pipe and a deep aeration pipe juxtaposed thereto (the invention as defined in claim 4).
The above-mentioned light bentonite slurry is a muddy water having a specific gravity of 1.05 to 1.20 obtained by adding, for example, a powder of calcium carbonate having an average grain diameter of about 10 to 20 microns, as a filler, to a bentonite slurry. This light bentonite slurry is mixed with, for example, a lost circulation inhibitor (LCM) as a filling-up material serving as a core at the time of formation of a mud cake for blocking a void and a crack formed in a rock-bed. Owing to the presence of the filler and the filling-up material, the mud cake is surely and firmly formed at the void and the crack of the rock-bed, thereby ensuring a stable air-tightness.
The above-mentioned heavy bentonite slurry is a suspension obtained by mixing, for example, a bentonite slurry with a high specific gravity fine powder such as barite and hematite as a load adjusting material in such a stable condition that it may hardly precipitate. The heavy bentonite slurry is a muddy water having a specific gravity of about 1.2 to 2.0. Owing to the heavy bentonite slurry having a high specific gravity, the bentonite slurry in the rock-bed can surely be held in a dual structure having a light bentonite slurry as an upper layer, and a mud cake can smoothly be formed at the top end part of the rock-bed cavity, which is a weak part where the void and the crack are hardly blocked up, by closely contacting the light bentonite slurry with the top end part easily.
It is preferred that in order not to lose activity by maintaining a stable state for a long period of time, the bentonite slurry is added with a dispersing agent such as phosphate, lignite, styrensulfonic acid/maleic anhydride copolymer and polyacrylic acid in accordance with necessity.
The light bentonite slurry is, in general, inexpensive compared with the heavy bentonite slurry which is expensive. Therefore, due to the dual structure in which the upper layer of the heavy bentonite slurry is replaced with light bentonite slurry, economic efficiency can be obtained by reducing the quantity of use of the heavy bentonite slurry. In addition, economical maintenance management can be obtained by supplementing the reduced quantity of the bentonite slurry in the rock-bed cavity with the inexpensive light bentonite slurry.
Moreover, according to the compressed gas storage tank of the present invention, since the compressed gas pipe for feeding a compressed gas is open downward from the top part of the rock-bed cavity, the bentonite slurry can fully be filled in the cavity of the rock-bed while completely allowing air and the like to escape from the cavity of the rock-bed, and in addition, the void and the crack of the top end part of the rock-bed cavity, at which part the stored gas tends to escape, can surely and economically be blocked by closely contacting the inexpensive light bentonite slurry mixed with a filling-up material with the top end part of the rock-bed cavity.
In the case where the light bentonite slurry is forcibly fed into the rock-bed cavity, in which the bentonite slurry is filled, through the compressed gas pipe, the light bentonite slurry is fed into the rock-bed cavity from the top part to the ceiling part of the rock-bed cavity with pressure applied thereto while pushing out the bentonite slurry already filled therein and therefore, blocking of the void and the crack formed in the rock-bed is enhanced by efficiently entering the light bentonite slurry into the void and the crack with the pressure to form the mud cake. Even if the light bentonite slurry is reduced by being infiltrated into the rock-bed, the function for blocking the void and the crack can be maintained continuously and economically by supplementing the inexpensive light bentonite slurry through the compressed gas pipe easily.
Moreover, according to the compressed gas storage tank of the present invention, in the case where the vertical shaft has associated facilities such as a reverse osmosis membrane water-generating pipe and a deep aeration pipe juxtaposed thereto, economic efficiency can be obtained by effectively utilizing the vertical shaft formed by excavating the underground deep into the rock-bed cavity.